1. Field of the Invention
This invention generally relates to optical communication interfaces and, more particularly, to a photodiode demultiplexer.
2. Description of the Related Art
As noted in Wikipedia, multi-mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building. Typical multimode links have data rates of 10 Mbit/s to 10 Gbit/s over link lengths of up to 600 meters. The equipment used for communications over multi-mode optical fiber is much less expensive than that for single-mode optical fiber. Typical transmission speed and distance limits are 100 Mbit/s for distances up to 2 km (100BASE-FX), 1 Gbit/s to 220-550 m (1000BASE-SX), and 10 Gbit/s to 300 m (10 GBASE-SR).
Because of its high capacity and reliability, multi-mode fiber (MMF) is generally used for backbone applications in buildings. An increasing number of users are taking advantage of fiber closer to the user by running fiber to the desktop. Standards-compliant architectures such as Centralized Cabling and fiber to the telecom enclosure offer users the ability to leverage the distance capabilities of fiber by centralizing electronics in telecommunications rooms, rather than having active electronics on each floor.
Multi-mode fiber has higher “light-gathering” capacity than single-mode optical fiber. In practical terms, the larger core size simplifies connections and also allows the use of lower-cost electronics such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs) which operate at the 850 nm and 1300 nm wavelength. Single-mode fibers used in telecommunications operate at 1310 or 1550 nm and require more expensive laser sources. Single mode fibers exist for nearly all visible wavelengths of light. However, compared to single-mode fibers, the multi-mode fiber bandwidth-distance product limit is lower. Because multi-mode fiber has a larger core-size than single-mode fiber, it supports more than one propagation mode; hence it is limited by modal dispersion, while single mode is not.
The LED or VCSEL light sources sometimes used with multi-mode fiber produce a range of wavelengths and these each propagate at different speeds. In contrast, the lasers used to drive single-mode fibers produce coherent light of a single wavelength. This chromatic dispersion is another limit to the useful length for multi-mode fiber optic cable. Multi-mode fibers may have a higher numerical aperture, depending on the indices of the core and cladding, which means they are potentially better at collecting light than single-mode fibers. Because of the graded index of the core of the MMF, the numerical aperture (NA) at any given point in the core is actually a function of radial distance from the center, i.e. NA(r), such that NA goes to 0 at the core/cladding interface. The larger core size eases alignment tolerances and permits a more efficient capture of light as compared to single-mode fiber. Due to the modal dispersion in the fiber, multi-mode fiber has higher pulse spreading rates than single mode fiber (SMF), limiting multi-mode fiber's information transmission capacity. The chromatic dispersion is also much higher in MMF near 850 nanometers (nm), at about −100 ps/nm-km, vs. SMF near 1310/1550 nm, at about zero to <+20 ps/nm-km, depending on the fiber type. So the higher pulse spread, while typically dominated by the modal delay component, is also partially due to chromatic dispersion, especially for the highest grades of MMF (e.g., OM4). However, the greater light gathering capability of multi-mode fiber makes it attractive for use in commercial applications where a wider range of tolerances creates imperfect optical alignment and dispersed light beams.
The vertical-cavity surface-emitting laser (VCSEL) is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers which emit from surfaces formed by cleaving the individual chip out of a wafer. The VCSEL has many potential advantages over the edge-emitting lasers. Its design allows chips or dies to be manufactured and tested on a single wafer. Large arrays of devices can be created exploiting methods such as flip-chip optical interconnects and optical neural network applications to become possible. In the telecommunications industry, the VCSEL's conical beam profile is desirable for coupling into optical fibers, as compared to the ellipsoidal beams typically output from edge-emitting laser. However, with these advantages come a number of problems particularly in the fabrication and operation at high powers.
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. The conventional solar cell used to generate electric solar power is a large area photodiode. Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays), or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode use a PIN junction rather than the typical p-n junction to increase the speed of response. A photodiode is designed to operate in reverse bias.
As the desired aggregate bandwidth of optical devices increases, either the throughput per fiber interface or the total number fiber interfaces must increase. The first solution requires expensive parts and complicated alignment procedures. The latter solution requires a larger interface port and a cable with more fibers, increasing the cable weight, size, and cost.
Conventional multi gigabit per second (Gbps) free-space optical interconnects use fiber-based transceivers to launch into either conventional or adaptive lens systems to produce highly collimated beams for use over distances of hundreds of meters to hundreds of miles. Such a system typically uses high-power lasers, external modulators, and active control over beam steering and shape to achieve high performance over far distances. However, the sheer complexity, size, and cost of such systems make them impossible to utilize for very high volume, very short reach consumer applications. These applications then typically resort to either the use of cables to make a physical connection, or increasingly use omnidirectional wireless standards, such as WiFi, Bluetooth, etc., which have a number of disadvantages, such as low throughput (compared to 1 Gbps) and the requirements of compensating for multipath interference and competition between devices for use of a limited bandwidth space, which arise due to their omnidirectionality and their use of radio frequency radiation. Some approaches also multicast optical signals using highly-divergent beams or beaconing structures.
Wavelength Division Multiplexing (WDM) is the transmission of multiple separately encoded channels at distinct wavelengths through the same waveguide or component. Current WDM demultiplexers are typically stand-alone devices with fiber inputs/outputs. Common approaches typically utilize either arrayed waveguide gratings, free-space gratings+micro-electromechanical (MEMS) devices, or other dispersive elements, in order to spatially disperse the different wavelengths and then couple them to/from separate waveguides. Alternatively, a filter and optical circulator are used to add or drop single wavelengths. These solutions tend to be large compared to a photodiode and require coupling to additional fibers, which results in excess loss and the full cost of the assembly. Consequently, these solutions are reserved for applications in which single-mode optics and fibers are employed, in which performance requirements dominate over cost due to the comparatively low number of links, compared to MMF link environments.
MMF links between optical modules, or within an optical cable, conventionally use each fiber for transmission in only one direction. This is because the use of fiber couplers after launch into the fiber introduces high optical losses while also potentially coupling large amounts of power back into the VCSEL, which cannot use optical isolators due to the multimode nature of the system. A large number of fibers adds weight, cost, and complexity.
The attenuation coefficient is a quantity that characterizes how easily a material or medium can be penetrated by a beam of light. A large attenuation coefficient means that the beam is quickly “attenuated” (weakened) as it passes through the medium, and a small attenuation coefficient means that the medium is relatively transparent to the beam. Attenuation coefficient is measured using units of reciprocal length. The attenuation coefficient is also called linear attenuation coefficient, narrow beam attenuation coefficient, or absorption coefficient.
The attenuation coefficient describes the extent to which the intensity of an energy beam is reduced as it passes through a specific material. This might be a beam of electromagnetic radiation. A small linear attenuation coefficient indicates that the material in question is relatively transparent, while a larger value indicates greater degrees of opacity or non-transparency. The linear attenuation coefficient is dependent upon the type of material and the energy of the radiation. Generally, for electromagnetic radiation, the higher the energy of the incident photons and the less dense the material in question, the lower the corresponding linear attenuation coefficient will be.
The measured intensity (I) transmitted through a layer of material with thickness x is related to the incident intensity I0 according to the inverse exponential power law that is usually referred to as Beer-Lambert law:I=I0e−αx,
where x denotes the path length. The attenuation coefficient (or linear attenuation coefficient) is α.
The Half Value Layer (HVL) signifies the thickness of a material required to reduce the intensity of the emergent radiation to half its incident magnitude. The attenuation factor of a material is obtained by the ratio of the emergent and incident radiation intensities I/I0.
When a narrow (collimated) beam of light passes through a substance, the beam will lose intensity due to two processes: The light can be absorbed by the substance, or the light can be scattered (i.e., the photons can change direction) by the substance. Just looking at the narrow beam itself, the two processes cannot be distinguished. However, if a detector is set up to measure light leaving in different directions, or conversely using a non-narrow beam, one can measure how much of the lost intensity was scattered, and how much was absorbed.
In this context, the “absorption coefficient” measures how quickly the beam would lose intensity due to the absorption alone, while “attenuation coefficient” measures the total loss of narrow-beam intensity, including scattering as well. “Narrow-beam attenuation coefficient” always unambiguously refers to the latter. The attenuation coefficient is always larger than the absorption coefficient, although they are equal in the idealized case of no scattering.
It would be advantageous if the number of fibers required for the same aggregate bandwidth could be reduced without requiring the use of optical modulation schemes.